Principles of Astronomy Exam 3

The main reason that the night sky is dark is that stars are generally so far away.

False; The fact that the night sky is dark tells us that we can only observe light from objects in our observable horizon, nothing past that.

Which kind of star is most likely to be found in the halo?

M star

Which of the following does inflation help to explain?

the origin of galaxies

When the universe was 380,000 years old, its thermal radiation spectrum consisted mostly of ...

visible and infrared photons

Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?

white dwarf supernovae

Which of these objects has the largest radius?

1.2 M sun white dwarf

Which of these objects has the smallest radius?

1.2 M sun white dwarf

What is the typical percentage (by mass) of elements other than hydrogen and helium in stars that are forming right now in the vicinity of the Sun?

2%

What is a nova? Describe the process that creates a nova and what a nova looks like.

A Nova is the dramatic brightening of a star that lasts for a few weeks and then subsides; it occurs when a bust of hydrogen fusion ignites in a shell on the surface of an accreting white dwarf in a binary star system. It looks a bright shine for a few glorious weeks and is recognized as a nova.

In what sense is a black hole like a hole in the observable universe? Define the event horizon and Schwarzchild radius, and describe the three basic properties of a black hole?

A black hole is like a hole in the observable universe as we can not see inside it. This is due to the event horizon, which is the area of a black hole where escape velocity exceeds c and nothing can escape. The Schwarzschild radius is the radius of the event horizon. The basic properties are mass, electric charge, and angular momentum.

Describe the mass, size, and density of a typical neutron star. What would happen if a neutron star came to your hometown?

A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star supernova. It resembles a giant atomic nucleus that is 10 kilometers in radius, and it has more mass than the Sun does. Neutron stars are one of the end points of the stellar evolution and are the final states of stars that begun their lives with considerably more mass than the Sun. Neutron stars are pulsars (pulsating radio sources) if they rotate sufficiently rapidly and have strong enough magnetic fields. If a neutron star came even close to Earth, let alone our town, we would all end up splattered across the face of the star.

Which of these neutron stars must have had its angular momentum changed by a binary companion?

A pulsar that pulses 600 times per second

What is degeneracy pressure, and how is it important to the existence of white dwarfs and neutron stars? What is the difference between electron degeneracy pressure and neutron degeneracy pressure?

A type of pressure that arises when subatomic particles are packed as closely as the laws of quantum mechanics allow. Electron degeneracy pressure comes from closely packed electrons whereas neutron degeneracy pressure comes from neutrons

If we see a nova, we know that we are observing...

A white dwarf in a binary system

What is an accretion disk? Describe how the accretion disk provides a white dwarf with a new source of energy that we can detect from earth.

Accretion disk is a rapidly rotating disk of material that gradually falls inward as it orbits a star like object. You find them in close binary systems because white dwarfs can gradually gain mass if its companion is a main sequence or giant star. The difference in gas particle speeds lead to friction that removes orbital energy from the inner region, and the loss of energy means that gas gradually spirals inward and eventually settles onto the white dwarf.

Which of these binary systems is most likely to contain a black hole?

An X-ray binary containing an O star and another object of equal mass.

What is antimatter? How were particle-antiparticle pairs created in the early universe? How were they destroyed?

Antimatter is an electron that is negatively charged. The early universe was so hot that photons could transform themselves into matter. They were destroyed by running into each other.

What causes a white dwarf supernova? Observationally, how do we distinguish white dwarf and massive star supernovae?

As a white dwarfs mass reaches 1.4Msun, its temperature rises enough to allow carbon fusion to begin. Carbon fusion ignites almost instantly throughout the white dwarf, creating a carbon bomb detonation similar to the helium flash in low mass red giants but releasing far more energy. The white dwarf explodes completely in what is called a White Dwarf Supernova. The observational difference is that a white dwarf supernova rapidly fades quickly during the first few weeks then decline more rapidly after. More massive stars are more complicated.

Why do virtually all the galaxies in the universe appear to be moving away from our own?

Because expansion causes all galaxies to be moving away from nearly all others.

What do we mean by the singularity of a black hole? How do we know that our current theories are inadequate to explain what happens at the singularity?

Because nothing can stop the crush of gravity in a black hole, all the matter that forms a black hole should ultimately be crushed to an infinitely tiny and dense point in the black holes center. We call this point a singularity. Both of the scientific claims make sense scientifically but oppose each other which means they are inadequate.

Why do disk stars bob up and down as they orbit the galaxy?

Because the gravity of other disk stars always pulls them toward the disk

Briefly describe the characteristics that distinguish disks stars from halo stars.

Disk population: contains both young and old stars all of which have heavy-element proportions of about 2% like our Sun Halo stars: consist of stars in the halo and bulge. Stars are always old and low in mass

What would happen if the sun suddenly became a black hole without changing its mass?

Earths orbit would not change

The white dwarf star at the center of the Helix planetary nebula has a mass 3 times the mass of the Sun.

False The maximum mass of a white dwarf is the Chandrasekhar mass, about 1.4 solar masses.

We can detect black holes with X ray telescopes because matter falling into a black hole emits x rays after it smashes into the event horizon.

False, black holes do not have surface for material to smash into. The X-ray emission comes from gas as it falls toward the event horizon and heats up

The Sun's velocity around the Milky Way tells us that most of our galaxy's dark matter lies near the center of the galactic disk.

False. The Milky Way's rotation curve remains flat well beyond the orbit of the Sun, indicating that the majority of the Milky Way's mass lies beyond the Sun's orbit.

The merger of two black holes forms a black hole with a smaller Schwarzschild radius than the original black holes.

False. The Schwarzschild radius would be larger than the original blackholes

We did not understand the true size and shape of the Milky Way Galaxy until NASA launched satellites into the halo, enabling us to see what the Milky Way looks like from the outside.

False. We can never launch any object with enough speed to reach escape velocity (speed of light)

In the distant past the cosmic microwave background consisted primarily of infrared light.

False;

According to the Big Bang theory, the cosmic microwave background was created when energetic photons ionized the neutral hydrogen atoms that originally filled the universe.

False; In order for photons to be able to travel unobstructed, the opposite had to happen - atoms that were ionized before became neutral.

Summarize each of the major links in the distance chain. Why are cepheid variable stars so important? Why are white dwarf supernovae so useful, even though they are quite rare?

For planets, can use radar ranging to determine distances. For closest stars, can use parallax to determine distances directly. For more distant stars within galaxy, use main-sequence fitting, technique uses known brightness of various classes of stars to turn main-sequence stars to standard candles. Does not work to find distance to other galaxies since most main-sequence stars too faint to be seen in other galaxies. In this case, use Cepheid variables, variable stars whose luminosities related to period of brightness variations. These stars very important since allowed to measure distances to nearby galaxies and played key role in Edwin Hubble's discoveries. Even Cepheid variables cannot tell distance to most galaxies, however, because we cannot see them. White dwarf supernovae much brighter and have nearly same peak luminosity. Although such supernovae rare, are bright enough to see to far reaches of universe-extremely powerful tool in determining distances.

How do we determine the Milky Way's mass outside the Sun's orbit?

From the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun.

What is Hubbles Law? What is Hubbles constant? Explain what we mean when we way that Hubble's constant is between 20 and 24 kilometers per second per million light years.

Hubble explained that galaxies far away from Earth move away with greater velocities; seen as the galaxys red shift in the spectrum of a galaxy ( v=Ho x d, Ho is hubbles constant) For every million light years of distance away from Earth, the speed of the galaxy receding from Earth is greater by 20 to 24 kilometers per second.

Explain how Hubble used Cepheid variable stars to prove that the Andromeda Galaxy lies beyond the bounds of the Milky Way.

Hubble observed the Andromeda galaxy and found objects that were dimming and brightening with continuous period and assumed them to be Cepheid variables. He found that the objects were brighter in the Andromeda galaxy and used the period-luminosity of Leavitt and was able to to find out their actual luminosity and calc the distances of those objects with the inverse of those objects with the inverse square law for light (outer reaches of Milky Way)

Suppose you are falling into a black hole. How will you perceive the passage of your time? How will outside observers see time passing for you? Briefly explain why your trip into a stellar mass black hole is likely to be deadly.

If you fell into a black hole time would be mere seconds until you disappear into oblivion, but to the others watching you seem to be in a dead stop on the event horizon. It would be deadly because gravity would pull your head and feet in opposite directions, while the pressure would crush you.

Where are the most of the Milky Way's globular clusters found?

In the halo

If you want to find a pulsar, you should look near the remanent of a supernova discovered by ancient Chinese astronomers.

It is sensible to look for pulsars near supernova remnants. An example of a pulsar found near the remnant of an observed supernova is the Crab pulsar, which was found near the Crab nebula. Note that massive stars aren't the only kind of stars that can explode in supernova explosions. White dwarfs in binaries can also explode in supernova explosions. These do not leave behind pulsars. So, not all supernova remnants will contain pulsars.

Viewed from a distance, how would a flashing red light appear as it fell into a black hole?

Its flashes would shift to the infrared part of the spectrum.

What would happen to a neutron star with an accretion disk orbiting in a direction opposite to the neutron star's spin?

Its spin would slow down.

Make a list of major eras in the history of the universe, summarizing the important events thought to have occurred during each era

Know little about the PLANCK ERA, when the 4 fundamental forces may have behaved as one. Gravity became distinct at the start of the GUT ERA, and electromagnetism and the weak force became distinct at he end of the ELECTROWEAK ERA. Matter particles destroy all the antimatter particles by the end of the PARTICLE ERA. Fusion of protons, and neutron into helium came to an end of the ERA OF NUCLEOSYNTHESIS ERA. Hydrogen nuclei captured all the free electrons, forming hydrogen atoms at the end of the ERA OF NUCLEI. Galaxies began to form at the end of the ERA OF ATOMS. THE ERA OF GALAXIES continues to this day.

What is the charge of an antiproton?

Negative

According to the big bang theory, most of the helium in the universe was created by nuclear fusion in the cores of stars.

No, according to the Big Bang Theory, most of the helium was created during the era of nucleosynthesis fusing hydrogen into helium, not inside the stars.

Scientists have just learned that there is a 10M Sun black hole lurking near Pluto's orbit.

No, if this is the case, we would be orbiting around it not the Sun. As your textbook points out, it is a common misconception that black holes suck things. For this to happen the Schwarzchild radii (the sphere of influence) must be large and most blackholes are not massive enough to have such large radii. If the Earth suddenly becomes a black hole, despite the darkness we will be in, we will still be orbiting the Sun.

Which of the following does not provide strong evidence for the Big Bang theory?

Observations of the amount of hydrogen in the universe

How do we know that pulsars are neutron stars? Are all neutron stars also pulsars? Explain.

Pulsars- the pulses coming from Cyngus were very surprising because no known astronomical object pulsated so regularly. In fact, the pulsations came at such precise intervals that they were nearly as reliable for measuring time as the most precise human-made clocks. The pulsations arise because the neutron star is spinning rapidly as a result of the conversation of angular momentum. As an iron core collapses into a neutron star, its rotation rate must increase as it shrinks in size.We know that pulsars must be neutron stars because no other massive object could spin that fast.

What is the shape of the Milky Way's halo?

Spherical

What do we mean by a standard candle? Explain how we can use standard candles to measure distances.

Standard candles are like measurements tools that have a known true luminosity without finding apparent brightness and distance. With this we can use it to find distance of other stars not measurable. I.e. A sun like star to measure distance Less than 1000 ly but for greater distance. We use white dwarf super novae since they explode once reaching 1.4 sm limit, they should have the same luminosity

Describe and contrast stellar orbits in the disk, halo, and bulge of our galaxy.

Stars in the disk orbit the galactic center in nearly circular orbits in the same direction in the same plane lying in the plane of the galaxy-Stars in bulge and halo of the galaxy, orbits the galactic center in elliptical orbits -Halo is anything outside the galaxy.

What happens to the electron speeds in a more massive white dwarf, and how does this behavior lead to a limit on the mass of a white dwarf? What is the white dwarf limit?

The electrons respond to compression by moving faster, which makes the degeneracy pressure strong enough to resist the greater force of gravity. The fact that electron speeds are higher in more massive white dwarfs leads to a fundamental limit to the maximum mass of a white dwarf. No white dwarf can have a mass greater than 1.4Msun, which is called white dwarf limit.

Explain what we mean by the Big Bang theory?

Theory that universe began in a single moment from incredible hot, tiny , dense collection of matter and radiation. And how expansion and cooling of unimaginable mixture of particles and photons led to the creation of our stars and galaxies.

What happens when a proton collides with an antiproton?

They convert into two photons

If your spaceship flew within a few thousand kilometers above the event horizon, you and your ship would be rapidly sucked into it.

This statement does not make sense. A circular orbit, even at a distance of a few thousand kilometers above the event horizon is perfectly stable. If you use the engines of the spaceship to put it on such orbit, the spaceship will not be sucked into the black hole.

The best way to search for black holes is to look for small black circles in the sky.

This statement does not make sense. We cannot distinguish between the objects that do not emit or reflect light and their black background using direct observations. However, the presence of the objects can be deduced from the indirect observations.

I observed a white dwarf supernova occurring at the location of a single (not binary) white dwarf.

This statement does not make sense. White dwarf supernovae arise from mass transfer from a companion star and thus cannot occur outside binary systems.

From your point of view, an object falling toward a black hole will never cross the event horizon.

This statement makes sense. According to Einstein's theory, from your point of view, the object takes forever to cross the event horizon. You' ll see how the object vanishes from view due to the huge gravitational red-shift of light.

What are the four forces that operate in the universe today? Why do we think there were fewer forces operating in the early universe.

Today's four forces - gravity, electromagnetism, strong force, weak force May have been unified early in time, becoming distinct as the universe expanded and cooled.

Halo stars orbit the center of our galaxy much faster than the disk stars.

True

After measuring a galaxy's redshift, I used Hubble's law to estimate its distance.

True as most astronomers use the law in reverse, first by getting the speed through redshift then calculate distance using Hubble law

Planets like Earth probably didn't form around the first stars because there were so few heavy elements back then.

True; Earth formed through the accretion of smaller, rocky objects made from heavy elements.

Cepheid variables make good standard candles

True; They follow a well-defined period-luminosity relationship. They are bright giants...luminous enough to see at great distances

What are grand unified theories? According to these theories, how many forces operated during the GUT era? How are these forces related to the four forces that operate today?

Unify the strong force with the electromagnetic and weak forces. 2 forces. These forces used to be unified.

How can we use orbital properties to learn about the mass of the galaxy? What have we learned?

We can use the sun's orbital velocity and its distance from the galactic center to determine the mass of our galaxy lying within the suns orbit. We need to consider the difference between the gravitational effects of mass within the suns orbit and those of mass beyond its orbit. We found that orbital speeds remain the same out to great distances from the galactic center, telling us that most of the galaxy's mass resides far from the center and is distributed throughout the halo.

Why can't current theories describe what happened during the Planck era?

We do not yet have a theory that links quantum mechanics and general relativity.

Why do we think that super novae should sometimes form black holes? What observational evidence supports the existence of black holes?

We think super novae should sometimes form black holes because as soon as the core exceeds the neutron star limit, gravity overcomes the neutron degeneracy pressure and the core collapses once again. This time, no known force can keep the core from collapsing into oblivion as a black hole. The only observational evidence of black holes since black hole emit no light, is the black holes influence of its gravitational influence on its surroundings.

Describe the mass, size, and density of a typical white dwarf. How does the size of a white dwarf depend on its mass?

White dwarfs have the mass of the sun (1Msun) compressed into the size of the earth. The density of white dwarfs is extremely high, for example a teaspoon of a white dwarf would weigh several tons. More massive white dwarfs are actually smaller in size compared to less massive ones that are larger in size.

Explain how the presence of a neutron star can make a close binary star system appear to us as an X-ray binary. Why do some of these systems appear to us as X-ray bursters?

X-Ray Binary-intense x-ray emission, close binaries that contain accreting neutron stars. X-ray bursters- accreting neutron stars sporadically erupt with a pronounced spike in luminosity. Because these eruptions release energy primarily in the form of X-rays we call them x-ray bursts, and systems that produce them are known as X-ray bursters.

Which of these black holes exerts the weakest tidal forces on an object near its event horizon?

a 10^6 MSun black hole

We determine the distance of a cepheid by measuring...

its parallax